A → color index correction applied to the photometric magnitudes
and colors of a distant galaxy to compensate for the "reddening" of the galaxy due
to → cosmological redshift. K correction is intended
to derive the magnitudes in the → rest frame of the galaxy.
Typically it is given as K(z) = az + bz2, where a and b
depend on galaxy types. Conversely, one may deduce the
redshift of a galaxy by its colors and a K-correction model.

The term K correction, probably stems from the K-term used by
C. W. Wirtz (1918, Astron. Nachr. 206, 109), where K stands for Konstante,
the German word for constant. The K-term was a constant offset in the
redshift applied to diffuse nebulae in that epoch (source: A. L. Kinney, 1996, ApJ 467, 38);
→ correction.

K star

ستاره‌ی ِ K

setâre-ye K (#)

Fr.: étoile de type K

An orange-red star of → spectral type K with a surface
temperature of about 3600-5000 K. The spectra of K stars are
dominated by the H and K lines of calcium and lines of neutral iron
and titanium, with molecular bands due to cyanogen (CN) and titanium
dioxide (TiO). Examples are → Arcturus
and → Aldebaran.

The hypothesis of the origin of the solar system proposed first by Kant
(1755) and later by Laplace (1796). According to this hypothesis, the solar system
began as a nebula of tenuous gas. Particles collided and gradually, under the influence
of gravitation, the condensing gas took the form of a disk. Larger bodies formed,
moving in circular orbits around the central condensation (the Sun).

Named after the German prominent philosopher Immanuel Kant (1724-1804) and the French great
mathematician, physicist, and astronomer Pierre-Simon Marquis de Laplace (1749-1827);
→ hypothesis.

kaon

کاءون

kâon

Fr.: kaon

Any of a group of four short-lived → mesons
distinguished by a → quantum number
called → strangeness.
Also called K meson and denoted K. They are positive, negative, or neutral
and have a mass of about 495 MeV/c (about 970 times that of an
→ electron).

A process based on the effects of → opacity (κ) that drives the
→ pulsations
of many types of variable stars. Consider a layer of material within a
star and suppose that it undergoes inward contraction. This inward
motion tends to compress the layer and increase the density. Therefore
the layer becomes more opaque (See also → partial ionization zone).
If a certain amount of flux comes from
the deeper layers it gets stuck in the high κ region. The energy
accumulates and heat builds up beneath it. The pressure rises below
the layer, pushing it outward. The layer expands as it moves
outward, cools and becomes more transparent to radiation. Energy can
now escape from below the layer, and pressure beneath the layer
diminishes. The layer falls inward and the cycle repeats. The
κ mechanism is believed to account for the pulsations of several
star families, including → Delta Scuti stars,
→ Beta Cephei variables, → Cepheids,
and → RR Lyrae stars
(See Baker & Kippenhahn, 1962, Zeitschrift für Astrophysik 54, 114). Same as
κ effect and → valve mechanism.
See also → gamma mechanism.

A way of measuring a civilization's technological advancement based
upon how much usable energy it has at its disposal.
The scale was originally designed in 1964 by the Russian astrophysicist
Nikolai Kardashev (who was looking for signs of extraterrestrial life
within cosmic signals). It has three base classes, each with an energy
disposal level: Type I, Type II, and Type III.
Type I designates a civilization that is capable of controlling the total energy of
its home planet (1016 watts).
Type II is an interstellar civilization, capable of harnessing the total energy
output of a star (1026 W).
And Type III represents a galactic civilization, capable of inhabiting and harnessing
the energy of an entire galaxy
(1036 W).
The scale has since been expanded by another four.
Type 0 is civilization that harnesses the energy of its home planet, but not to its full
potential. The Earth civilization is currently at about 0.73 on the Kardashev
scale.

The scale was originally designed in 1964 by the Russian astrophysicist
Nikolai Kardashev (1932-); → scale.

Keeler Gap

گاف ِ کیلر

gâf-e Keeler

Fr.: lacune de Keeler

In the system of → Saturn's rings, the
gap near the outer edge of the → A ring.
It has a width of 35 km and lies at a distance of 136,530 km from the
center of → Saturn.

The first achromatic eyepiece consisting of a convex lens and a
plano-convex lens. The convex surfaces are turned toward one another.

Named after the inventor Carl Kellner (1826-1855), a German engineer and optician;
→ eyepiece

kelvin (K)

کلوین

kelvin (#)

Fr.: kelvin

The basic unit of thermodynamic temperature in the international system of units
(→ SI units). The kelvin is also the fundamental unit of the
→ Kelvin scale.
One kelvin represents the same temperature difference as one degree
→ Celsius. It was previously called the degree Kelvin (°K).

Named after the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907),
one of the most influential scientists of the 19th century.

Kelvin scale

مرپل ِ کلوین

marpel-e Kelvin

Fr.: échelle de Kelvin

A temperature scale, redefined in 1954, in which the zero point is equivalent to
-273.16 °C. This fundamental fixed point, based on
the → triple point of water, is considered to be the lowest
possible temperature of anything in the Universe.
Also known as the absolute temperature scale.

A transformation whose only final result is to transform into work
heat extracted from a source which is at the same temperature is
impossible. Kelvin's postulate is a statement of
the → second law of thermodynamics and is
equivalent to → Clausius's postulate.

After the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907)
and the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894),
who made important contributions to the thermodynamics of gaseous systems;
→ contraction.

Kelvin-Helmholtz instability

ناپایداری ِ کلوین-هلمهولتس

nâpâydâri-ye Kelvin-Helmholtz (#)

Fr.: instabilité de Kelvin-Helmholtz

An → instability raised when there is
sufficient velocity difference across the interface between two uniformly moving
→ incompressible fluid layers, or when velocity
→ shear is present within a continuous fluid.

The characteristic time that would be required for a contracting spherical cloud of gas
to transform all its → gravitational energy
into → thermal energy. It is given by the relation:
tKH ≅ GM2/RL, where G is
the → gravitational constant, M is the mass of the
cloud, R the initial radius, and L the → luminosity.
The Kelvin-Helmholtz time scale determines how quickly a pre-main sequence star contracts
before → nuclear fusion starts.
For the Sun it is 3 x 107 years, which also represents the time scale on which
the Sun would contract if its nuclear source were turned off. Moreover, this
time scale indicates that the gravitational energy cannot account for the
solar luminosity. For a → massive star of M = 30 Msun,
the Kelvin-Helmholtz time is only about 3 x 104 years.

One of several layers in the Earth's ionosphere occurring at 90-150 km
above the ground. It reflects medium-frequency radio waves whereby
radio waves can be propagated beyond the
horizon.

Named after the American electrical engineer Arthur Edwin Kennelly
(1861-1939) and the English physicist Oliver Heaviside (1850-1925),
who independently predicted the existence of the reflecting layer in
1902; → layer.

Kepler

کپلر

Kepler (#)

Fr.: Kepler

Johannes Kepler (1571-1630), a German mathematician and astronomer and
a key figure in the 17th century astronomical revolution.
He discovered that the Earth and planets travel about the Sun in elliptical orbits;
gave three fundamental laws of planetary motion, and also did important
work in optics and geometry.

Kepler problem

پراسه‌ی ِ کپلر

parâse-ye Kepler

Fr.: problème de Kepler

1) Given the trajectory of a particle moving in a → central force
field, determine the
law governing the central force.
2) Inversely, considering a central force -k/r2, determine the
trajectory a particle moving in the field will take.